Why is the "render engine" selection menu missing?
Short version:


check first a build from builder.blender.org
then

verify source modifications
verify build settings

fix your modifications or build settings.


When you see weird behavior in own builds for which you also have changed source always check a build from builder.blender.org . If the unexpected behavior doesn't show up with such a nightly test-build, then you should review your modifications and build settings as they are apparently the cause for the behavior.

For building settings and troubleshooting of builds, refer to the build instructions for your own platform and build system. There are trouble shoot sections for each way.

If on the other hand the unexpected behavior is replicable with such a build, then you likely have found a bug, at which point you should be reporting that to projects.blender.org